1. Field of the Invention
The invention relates to an inductive length measuring system which, by scanning a scale with a graduation of periodically variable reluctance and a coil system in a linear arrangement, can detect information relating to the position and/or the movement of the coil system with reference to the scale.
2. The Prior Art
Such a length measuring system is disclosed in DE 19803249 A1.
A general comparison of known measuring systems yields the following result:
Main features of optoelectronic measuring systems: High accuracy for fine graduation periods, extremely sensitive to contamination, low shock-vibration loading.
Main features of magnetic measuring systems: Mean accuracy through graduation accuracy and interpolation error within the graduation period (harmonic content and signal deviations from one period to the other); polarized scale attracts magnetic particles and can be erased or damaged by external magnet noise fields.
Main features of inductive measuring systems: Many designs are known, generally a very robust design, extremely slight influence by temperature, based on the transformer principle, the transmission between primary and secondary coils being influenced by an element which moves relative to the coils.
Some design examples for inductive measuring systems:
The transducer of INDUCTOSYN (trademark) type comprises two elements, a scale and carriage, both of which have active planar windings, meandering on the primary and secondary sides, in the measuring surface. The device has a high accuracy, but requires a large coupling surface between the two elements because of its scanning principle. It functions in low carrier frequency ranges, and this limits the traversing speed and is very complicated in terms of design.
Inductive probes are cylindrical differential transformers comprising a primary coil, coupled to two concentrically wound secondary coils connected in opposition, and a core with a marking of different reluctance. The device transforms the relative position of the plunger core coils into an approximately linear output signal for a defined measuring range. High accuracies can be achieved for small measuring strokes, but the device is not suitable for larger measuring strokes and dynamic applications.
EP 0557608 B1 describes a spiral coil structure which is built in a multilayer metal insulating coating onto a soft magnetic or hard magnetic substrate using thick film technology. A measuring device functions according to the principle only in frequency ranges which do not bring the coil substrate into nonlinear magnetic regions, and is not suitable for high accuracies, owing to the detection of phase shift in the downstream resonant circuits.
The applied measuring principle for a type of xe2x80x9cinductive potentiometerxe2x80x9d is described by the same applicant in DE 19813497 A1. In that case, the relatively coarse accuracy which can be achieved is strongly influenced by tiltings of the core with respect to the coil plane.
EP 0805339 A1 describes a measuring device which functions according to a similar functional principle as described in the previous example. The device uses a planar multi-ply transformer coil arrangement for scanning a toothed measuring wheel. This arrangement comprises a primary coil in a plane, and two secondary coils offset from one another in the measuring direction. The individual secondary coils form either two measuring channels, which permit the direction to be detected by the local phase offset of the amplitude-modulated signals, or one measuring channel, the coils being connected differentially. In the first design variant (which exhibits a non-differential arrangement), the signals generated, with a weak degree of modulation, are strongly influenced by parasitic effects such as temperature, geometrical tilting of the coils with respect to the measuring wheel etc., and in the second design the detection of the direction of movement is possible only for high speeds (for slow point-to-point positioning the phase shift induced by speed tends to zero). This printed publication does not consider possible inductive interactions of a plurality of coils in one and the same structure. As also shown in the examples of the singly digitizing electronic evaluation system, the device described is suitable only for coarse detection of movement.
Inductive, absolutely operating position transducers are described in DE 19803249 A1 (Mitutoyo company). This device is designed chiefly for calliper rules. A plurality of measuring tracks arranged parallel to one another are scanned for the purpose of absolute position detection. The measuring device comprises a metal-structured scale, embedded in the body of a calliper rule, and a coil arrangement operating differentially in principle is accommodated in the carriage, which is guided accurately in relation thereto. A periodic metal graduation of a coil subsystem is scanned only with regard to the incremental track (finest measuring track). This planar system chiefly comprises a field coil and two receiver coil channels, which are inductively coupled to the field coil and can detect the position via the measuring graduation in the case of a relative movement. The two receiver coil channels are arranged in a phase-shifted fashion relative to one another (geometrically offset from one another) in order to detect the direction of movement.
The receiver coils, which comprise for each channel a plurality of differentially connected individual turns, are surrounded in the structural plane by the field coil turns. The magnetic field generated by the field coil does not have a uniform distribution over the entire inner coil region, but is substantially stronger in the vicinity of its turns and decreases in the direction of the middle of the coil. It is geometrically impossible because of this effect to place two differential, offset coils within an emitter framework in conjunction with the same field form and field strength. This means that, at least one of the two measuring channels, which are connected (in a simplified representation) as two differential receiver coils connected in opposition, are not arranged symmetrically relative to the emitter framework and are flowed through by different magnetic field strengths. In the case of a relative movement in the measuring direction with respect to the scale, the result after subtraction of the induced voltages is that the modulated useful signal does not oscillate about xe2x80x9czeroxe2x80x9d, but fluctuates about a value which is proportional to the static field strength difference in the two receiver surfaces. It is virtually impossible for this value, denoted as signal offset, to be completely adjusted in a downstream electronic evaluation system, since its amplitude is partly influenced by secondary effects of movement such as the coil-scale separation or relative tiltings, and it is therefore not constant for an overall measuring operation.
The author of this patent specification recognizes the problem (page 10, paragraph 25), but the design proposal is not effective, since although placing the asymmetric receiver winding pairs further away from the emitter windings in the middle of the emitter coils does bring the latter into a region where the exciter field and its gradients are weaker, there is also a consequent reduction in the induced useful signal, and so the ratio of useful signal to signal offset remains unfavorable, as before.
It is the object of the invention to combine the advantages of the optoelectronic length measuring devices, which have high accuracies and high achievable resolutions in the range of xe2x89xa61 xcexcm, with the advantages of the inductive devices, which have a high degree of robustness and stability with respect to environmental influences.
The above-mentioned object is accomplished by an inductive measuring device for detecting position, comprising a coil structure and a measuring scale having at least one graduation of variable reluctance or conductivity, wherein the coil structure comprises a multi-ply arrangement of a combination of coils in the form of planar spiral windings or turns having a plurality of receiver pairs, each pair having two differentially connected receiver elements which can be interconnected for the purpose of signal generation for each of at least two measuring channels, and wherein at least one emitter element is provided which is inductively coupled to the receiver elements as a function of the relative position in the measuring direction relative to the measuring scale, whereby at least one output signal is generated which is compensated for offset and/or sinusoidal shape and/or amplitude.
Owing, in particular, to the high accuracy in the xcexcm region and to the high achievable system dynamics, the measuring device described below is suitable for process controllers in which machine elements have to be accurately positioned.
In addition to the previous advantages, particular mention is to be made of the high degree of miniaturization and the favorable costs of production technology for scanning sensors and the measuring scale.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to, and forming part of, this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated and described preferred embodiments of the invention.